In a lot of electronic devices, a DC-DC converter circuit is required for supply of a stable rated working voltage. The DC-DC converter circuit has a generally construction that comprises a transistor based switching unit, which generally adopts a metal oxide semiconductor (MOS) field effect transistor (FET), a comparator, a saw-tooth wave generation circuit, an output voltage detection circuit, a feedback differential amplification circuit, and a reference voltage signal generation circuit. The operation of the DC-DC converter is that the output voltage detection circuit detects the voltage level of a DC output voltage and, in response thereto, generates a feedback signal that is fed through the feedback differential amplification circuit and the comparator to provide a gate control signal that controls the ON/OFF state of the transistor based switching unit in order to generate a stable DC output voltage at a voltage output terminal. Such a DC-DC converter has been commonly adopted in power supply circuits for liquid crystal display devices.
FIG. 1 of the attached drawings illustrates a circuit block diagram of a conventional power supply circuit for a liquid crystal display. The liquid crystal display, which is generally designated at 100, comprises a liquid crystal display panel 1, a gate driver 11, a data driver 12, and a logic control unit 13. These components/devices are operated with different working voltages. For a classic liquid crystal display 100, various working voltages of different levels are needed, including at least four different voltage levels, such as a gate switching-on voltage VGH, a gate switching-off voltage VGL, a data driving voltage VDD, a control logic circuit voltage Vlogic. All these working voltages are provided by a direct current supply circuit 200 and all these working voltages have different rated values. For example, the data driving voltage VDD is a working voltage of high voltage level and is provided by a boost-typed DC-DC converter.
Considering the DC-DC converter that provides the data driving voltage VDD as an example, as shown in FIG. 2, the DC-DC converter, which is generally designated with reference numeral 2, is supplied with a DC input voltage Vin flowing through a voltage supply circuit loop 201 consisting of an inductor element L and a forward-connected diode D and generates a DC output voltage Vout at a voltage output terminal N2. The voltage output terminal N2 is normally connected with a capacitor C serving as a filter.
The DC-DC converter 2 comprises a transistor based switching unit 21, which is a switching circuit composed of a MOS FET or power transistors of other types. The transistor based switching unit 21 has a drain that is connected to a node N1 between the inductor element L and the diode D, and a source that is electrically grounded. The transistor based switching unit 21 also has a gate that is electrically connected to a gate driver circuit 22.
A comparator 23 has a saw-tooth wave signal input terminal 23a, a differential signal input terminal 23b, and an output terminal 23c. The saw-tooth wave signal input terminal 23a receives a saw-tooth wave signal Vs from a saw-tooth wave signal generation circuit 24. The output terminal 23c of the comparator 23 is electrically connected to the gate driver circuit 22 to provide a gate control signal Vp to the gate driver circuit 22.
An output voltage detection circuit 25 is electrically connected to the voltage output terminal N2 to detect the voltage level of the DC output voltage Vout at the voltage output terminal N2, and in response thereto, generates a feedback signal Vfeb. The output voltage detection circuit 25 is composed of a first resistor R1 and a second resistor R2 that are connected in series to constitute a voltage divider circuit. A feedback node N3 between the first resistor R1 and the second resistor R2 provides a divided voltage signal, serving as the feedback signal Vfeb.
A feedback differential amplification circuit 26 has a feedback signal input terminal 26a, a reference voltage input terminal 26b, a differential signal output terminal 26c. The feedback signal input terminal 26a receives the feedback signal Vfeb from the output voltage detection circuit 25. The reference voltage input terminal 26b receives a reference voltage Vref generated by a reference voltage signal generation circuit 27. The differential signal output terminal 26c is electrically connected to the differential signal input terminal 23b of the comparator 23. Based on the feedback signal Vfeb and the reference voltage Vref received, the feedback differential amplification circuit 26 generates and feeds an error signal Verr through the differential signal output terminal 26c thereof to the differential signal input terminal 23b of the comparator 23. With such a DC-DC converter constituted by the above arrangement of the components/circuits/devices, a stable output voltage Vout can be obtained at the voltage output terminal N2 and the output voltage Vout is determined from the following equation: Vout=(1+R1/R2)Vref.
In some applications, such a conventional arrangement of the DC-DC converter works perfectly to supply the required rated voltage output for ordinary electronic devices. However, the known circuit of the conventional DC-DC converter is not satisfactory in view of the requirements for high precision, high environment durability, high stability, and low temperature drafting.
This is particularly true for liquid crystal displays. This is simply because the characteristics of a liquid crystal display are often affected by temperature change at the display panel of the liquid crystal display as well as the change of ambient temperature. For example, when the ambient temperature rises, the phase difference of the liquid crystal display panel is reduced and electric charges on the liquid crystal display panel are increased, leading to overcharging. This phenomenon influences the optic characteristics of the liquid crystal display panel, including the brightness, transmission, and gamma curve.
To overcome such a problem, conventionally, the data driving voltage VDD is increased, or the gate switching-on voltage VGH is reduced or lowered. This solution cannot effectively counteract the influence to the liquid crystal display panel caused by temperature changes. Further, this conventional technique cannot realize the temperature compensation operations of positive temperature coefficient or negative temperature coefficient according to the temperature changes by means of signal switching.
Various temperature compensation techniques are available in prior patent references. For example, US Patent Publication No. 2007/0085803A1 discloses a temperature compensation circuit for a liquid crystal display, wherein the temperature compensation circuit is realized by an operational amplifier, together with associated resistors and capacitors, which circuit is connected in series to a front stage of a common circuit for both a gate switching-on voltage (VGH) and a data driving voltage (VDD) of a liquid crystal display. This arrangement provides an effect of temperature compensation to certain extents, yet it is operated with a comparator that performs simple comparison between signals wherein the comparator compares the voltage levels of a detected ambient temperature and a data driving voltage (VDD) to generate a compensation voltage that is applied to a gate switching-on voltage supply circuit and a data driving voltage supply circuit. The regulation of the output voltage in this way is not precise. Further, the voltage regulation operation is concurrently carried out on both the gate switching-on voltage (VGH) and the data driving voltage (VDD) of the liquid crystal display without taking into consideration the different requirements existing between the gate switching-on voltage and the data driving voltage. Consequently, this solution is impractical in actual applications.
Another example is illustrated in U.S. Pat. No. 7,038,654, which also discloses a temperature compensation circuit for a liquid crystal display, which supplies a temperature signal obtained with a temperature sensor to a driver controller. The driver controller in turn provides a control signal that controls a reference voltage of an amplifier, and this, together with a step-up circuit, effects the regulation of an output voltage. This technique, although workable for temperature compensation, requires the change or adjustment of reference voltage and employment of digital technique to ensure realization of temperature compensation. This is not easy for practicing.
A further example is U.S. Pat. No. 6,803,899, which also discloses a temperature compensation circuit for a liquid crystal display, wherein a temperature signal obtained with a temperature sensor is used to regulate the voltage output with digital control technique, together with pulse width control technique. This solution also relies on digital control technique to realize temperature compensation and is thus difficult to practice.